1. Field of the Invention
This invention relates to an improvement in the production of polypropylene. More specifically, it relates to a process for the purification of propylene polymers produced by contact with a Ziegler Type catalyst in liquid phase in a C.sub.3 hydrocarbon medium.
2. Description of the Prior Art
The polymerization of propylene by contact with so-called Ziegler or Ziegler-Natta catalysts comprising essentially titanium trichloride and an aluminum alkyl compound is a well developed art. Commercial processes for production of polypropylene predominantly employ a liquid hydrocarbon medium of higher boiling point than propylene as diluent in the polymerization reaction. It is also known to conduct the polymerization in a medium consisting of propylene which may contain minor amounts of propane and possible small amounts of other hydrocarbon components.
The crude propylene polymers obtained in this way contain catalyst residues which may adversely affect the properties of the polymer. By "catalyst residues" is meant the catalyst and any decomposition products or derivatives thereof which may be formed during the polymerization.
The removal of these catalyst residues is usually termed "deashing" and the deashing efficiency is generally denoted by the percentage of titanium which is removed from the polymer during the purification. This is given by the expression
Deashing efficiency=100.times.(Ti.sub.initial -Ti.sub.final)/(Ti.sub.initial). PA1 (a) contacting a slurry of the polymer in a liquid C.sub.3 hydrocarbon medium with from 0.01 to 7%W (based on the liquid phase) of a C.sub.3 -C.sub.4 alcohol and with at least 0.1 mole of oxygen, or an equivalent amount of a peroxide, per gram atom of titanium present in the slurry, PA1 (b) washing the polymer with a liquid C.sub.3 hydrocarbon medium in the presence of from 0.01 to 20%W (based on the wash liquid) of an alcohol, and PA1 (c) recovering the washed polymer.
This definition is usually chosen because titanium can be determined more accurately than, for example, aluminum, and, moreover, the removal of other elements broadly follows that of titanium.
In addition to catalyst residues, the polymerization reaction product also contains some hydrocarbon-soluble propylene polymer components, usually referred to as atactic polypropylene, which it may be desirable to remove in whole or in part prior to recovery of the desired propylene polymer product.
Many different methods have been disclosed in the art for treating propylene polymerization reaction product for deactivation of catalyst components, removal of deactivated catalyst components and removal of undesired atactic polymer. A number of different methods are employed in commercial propylene polymerization processes.
Among the known methods, it is common to contact the total reaction product, which is a slurry of polypropylene particles in hydrocarbon diluent, with an alcohol to convert the active catalyst residue to catalytically inactive compounds of titanium and aluminum. It is also common to wash out the deactivated catalyst residues by contacting the slurry with an alcohol or with an aqueous medium or with a hydrocarbon.
It has also been disclosed to employ oxygen or compounds which liberate oxygen, such as organic peroxides, in the polymerization product work-up. A very early patent including such disclosures is British Pat. No. 840,861 to Petrochemicals Limited. This patent discloses subjecting a Ziegler polymerization product to the action of an oxidizing agent such as air or peroxide and also to the action of a liquid reagent for decomposing the catalyst. The liquid reagents disclosed are lower aliphatic alcohols or other organic hydroxy compounds such as glycols, as well as water, dilute aqueous acid or "other liquid reagents suitable for use in decomposing Ziegler catalysts". It is said that the oxidation treatment results in making the catalyst remnants removable by treatment with an alcoholic or other liquid reagent to a substantially greater extent and more easily than previously possible. A later disclosure of utilizing oxygen or oxygen-liberating compounds in the treatment of polyolefin reaction products is found in Canadian Pat. No. 821,610 to Montecatini. This patent is directed to purifying olefin polymers by contacting the crude polymer suspended in a hydrocarbon medium with an oxygen donor substance in the presence of a Lewis base so as to transform the catalyst residues into compounds which are soluble or finely dispersible in the hydrocarbon medium and which then can be removed from the polymer by filtration and washing with the same hydrocarbon used in preparing the polymer suspension. The Lewis bases disclosed are aromatic compounds, ethers, amines, olefins, nitrogen-containing heterocyclic compounds, etc. The examples of this patent disclose very effective removal of catalyst residues by a process in which a crude propylene polymerizate was filtered, the residue washed twice with boiling heptane, the polymer reslurried in heptane, dry air bubbled through the slurry for an hour at 92.degree. to 94.degree. C., and the polymer then washed five times with boiling heptane.